1. Field of the Invention
The present invention relates to a sheet conveying apparatus capable of causing a sheet to wait in a conveying route and to meet a following sheet and conveying them in a sheet bundle, a sheet processing apparatus including the sheet conveying apparatus, and an image forming apparatus including the sheet conveying apparatus.
2. Description of the Related Art
There is a known sheet processing apparatus for performing various kinds of sheet processes such as a binding process, a punching process, and a sorting process for sheets on which images are formed by an image forming apparatus such as a copying machine, a laser beam printer, and a facsimile. Conventionally, this type of sheet processing apparatus is provided with a portion (hereafter referred to as a buffering processing portion) in which a sheet is made to stay temporarily to avoid reduction in sheet processing productivity in a case of a process (e.g., the binding process) requiring a relatively long sheet processing time.
In such a sheet processing apparatus, the sheet is made to stay temporarily in the buffering processing portion and then conveyed downstream, with a following sheet conveyed from an upstream side in a sheet bundle of the plurality of sheets. In this way, the processing time for the sheet processing portion provided downstream is secured. In this manner, by causing the sheet to stay in the buffering processing portion provided to a conveying portion between the image forming apparatus and the sheet processing portion while the sheets are processed in the downstream portion and by not stopping discharge operation from the image forming apparatus, production capacity of a system in total is enhanced.
As disclosed in U.S. Patent Application Publication No. 2007/0075479 A1 and U.S. Patent Application Publication No. 2004/0175217 A1, for example, there is a sheet processing apparatus including a reversing-type buffering mechanism in which rear ends of the sheets are sorted by a switch member and the sheets are superimposed on each other a bundle while collision with a front end of a following sheet is prevented.
As illustrated in FIG. 18, a sheet processing apparatus in U.S. Patent Application Publication No. 2007/0075479 A1 has what is called a sheet buffering processing function for causing a plurality of superimposed sheets P to wait in a conveying route on an upstream side of a stapler 1132 and a saddle unit 1135. An operation flow of the buffering process is as follows.
When a rear end portion of a sheet P1 conveyed from a pair of inlet rollers 1102 is guided by and passes a lower face of a switch member 1114, a solenoid (not illustrated) is actuated and the switch member 1114 is turned down to guide the sheet P1 to a buffer path 1113. In this state, a pair of buffer rollers 1115 and 1116 are rotated reversely. As a result, the rear end of the sheet P1 enters the buffer path 1113 and then the sheet P1 is conveyed reversely until a front end of the sheet P1 arrives at a position B.
Then, the switch member 1114 is turned up so that a second sheet P2 can be conveyed toward the pair of buffer rollers 1116. At this time, a pair of reversing rollers 1112 is rotated at the time when a front end of the sheet P2 arrives at the position B to start conveying the sheet P1 toward the pair of buffer rollers 1116. With the front ends of the sheet P1 and the sheet P2 aligned with each other on a downstream side of the position B, the bundle of the two sheets is conveyed.
When a rear end portion of the sheet bundle of the sheets P1 and P2 passes the switch member 1114 similarly to the sheet P1, the switch member 1114 is turned down and the sheet bundle of the sheets P1 and P2 is conveyed to the buffer path 1113. In this way, the switch member 1114 is reciprocated every time the sheet passes to sequentially superimpose the sheets on each other to thereby bundle the sheets in the conveying route.
U.S. Patent Application Publication No. 2004/0175217 A1 is the sheet processing apparatus having a different buffering mechanism from U.S. Patent Application Publication No. 2007/0075479 A1.
As illustrated in FIG. 19, when a rear end of a sheet P1 passes a rear end holding member 2135 and arrives at a reversing point provided before a buffer roller 2124, the sheet P1 is returned to an upstream side (toward the rear end holding member 2135) by reverse rotation of the buffer roller 2124. At about the same time, the rear end holding member 2135 is separated from a lower conveying guide plate 2123b to open a rear end receiving portion 2136.
Then, the rear end holding member 2135 returns to an original position to press the sheet P1 against the lower conveying guide plate 2123b with a friction member of the rear end holding member 2135. Then, a second sheet P2 is sent in by a pair of inlet rollers 2121, passes over the rear end holding member 2135, and is conveyed by the buffer roller 2124 as well.
At this time, the sheet P1 is pressed against the lower conveying guide plate 2123b together with the sheet P2 by the buffer roller 2124 and tries to move toward a downstream side following the conveyed sheet P2. However, the sheet P1 does not move due to the friction member of the rear end holding member 2135.
Similarly to the sheet P1, when a rear end of the sheet P2 arrives at the reversing point, the sheet P2 is returned toward the upstream side (toward the rear end holding member 2135). Then, the sheet P2 is superimposed on the sheet P1 and pressed against the lower conveying guide plate 2123b by the friction member of the rear end holding member 2135.
Then, a third sheet (not illustrated) is sent and a rear end of the sheet passes the pair of inlet rollers 2121. The three sheets P1, P2, and P3 are nipped between a pair of upper first discharge rollers 2126a and a pair of lower first discharge rollers 2126b and the sheet bundle of these three sheets P1, P2, and P3 is conveyed to a downstream processing tray.
As described above, the prior-art sheet processing apparatus is of the buffering type in which the sheets are superimposed on each other in the bundle while the switch member for sorting the rear ends is reciprocated during intervals of the conveyed sheets. In this way, the apparatus provides satisfactory conveying performance and processing productivity.
However, in the buffering type in which the sheets are superimposed on each other in the bundle while the switch member is moved during the sheet interval for each sheet as in U.S. Patent Application Publication No. 2007/0075479 A1, the sheet interval when the sheet passes the switch member needs to be equal to or longer than a reciprocating time of the switch member. Therefore, it is difficult to adapt to a highly productive apparatus with short sheet conveying intervals.
Especially, because a thin sheet of 38 gsm to 52 gsm has a low mass per sheet and has low rigidity, discharge of the thin sheet to a stack tray is liable to be unstable. Therefore, to discharge such a thin sheet to the stack tray, a plurality of sheets needs to be bundled in the buffering processing portion before discharge so as to obtain a higher mass and rigidity to be discharged. However, as described above, the highly productive apparatus in which the sheets are conveyed at short intervals is difficult to adapt to the thin sheets of 38 gsm to 52 gsm.
Furthermore, in U.S. Patent Application Publication No. 2004/0175217 A1, because the reversing point is provided downstream of the rear end holding member 2135 for sorting of the rear ends, an upward opening operation control is performed so that the rear end portion is reliably conveyed to a lower of the rear end holding member 2135 at the time of reversal. In this way, the reciprocation for opening and closing the rear end holding member 2135 needs to be performed during the sheet interval for each sheet and therefore the apparatus is difficult to adapt to the highly productive apparatus similarly to U.S. Patent Application Publication No. 2007/0075479 A1.
As described above, in the prior-art sheet processing apparatus, the sheets are bundled while the switch member for sorting the rear ends is reciprocated during the sheet interval for each sheet and therefore there is a limit to processing velocity and it is difficult to increase productivity. Moreover, in the highly productive apparatus, it is difficult to buffer the sheets in a bundle. Therefore, it is difficult to discharge the plurality of thin sheets of 38 gsm to 52 gsm as a unit to improve stacking performance. As a result, the conveying performance and the stacking performance are difficult to improve.